Method for manufacturing a free-standing substrate made of monocrystalline semiconductor material

ABSTRACT

A method for manufacturing a free-standing substrate made of a semiconductor material. A first assembly is provided and it includes a relatively thinner nucleation layer of a first material, a support of a second material, and a removable bonding interface defined between facing surfaces of the nucleation layer and support. A substrate of a relatively thicker layer of a third material is grown, by epitaph on the nucleation layer, to form a second assembly with the substrate attaining a sufficient thickness to be free-standing. The third material is preferably a monocrystalline material. Also, the removable character of the bonding interface is preserved with at least the substrate being heated to an epitaxial growth temperature. The coefficients of thermal expansion of the second and third materials are selected to be different from each other by a thermal expansion differential, determined as a function of the epitaxial growth temperature or subsequent application of external mechanical stresses, such that, as the second assembly cools from the epitaxial growth temperature, stresses are induced in the removable bonding interface to facilitate detachment of the nucleation layer from the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/349,295 filed Jan. 22, 2003; now U.S. Pat. No. 6,964,914 andthis application is a continuation-in-part of U.S. patent applicationSer. No. 11/009,138 filed Dec. 13, 2004, now U.S. Pat. No. 7,071,029which is a division of U.S. patent application Ser. No. 10/446,604 filedMay 27, 2003, now U.S. Pat. No. 6,867,067 which is a continuation ofInternational Application No. PCT/FR01/03715 filed Nov. 26, 2001. Theentire content of these applications is hereby incorporated herein byreference thereto.

FIELD OF THE INVENTION

This invention relates to a method for manufacturing a free-standingsubstrate made of mono-crystalline semiconductor material, and inparticular of a wide band gap material.

BACKGROUND ART

A free-standing substrate is defined as a substrate, whose thickness issufficient that it carries itself without support. Reasonably, such asubstrate must thus have a thickness of at least 100 μm. However, inorder to be able to be manipulated in a manufacturing line without riskof breaking, it must generally be thicker.

By way of example, the commercially available free-standing substratescomprised of GaN or AlN have a thickness of 300 μm. Such free-standingsubstrates comprised of GaN or AlN are used in onto-electronic devicessuch as LEADS, lasers, sensors or in micro-electronic devices(transistors) or function in a high-temperature environment, or even inthe field of hyper frequency power or power electronics.

A first possibility for obtaining free-standing substrates can comprisefashioning them from a block of the material concerned by sawing andpolishing.

Unfortunately, at the present time, there is no manufacturing method forGaN or AlN ingots that can be used on an industrial development scale.

The article “Bulk and homo epitaxial GaN growth and characterization”,Porowski-S, Journal of Crystal Growth, Vol. 189-190, June 1998, pp153-158, describes a process for growing monocrystalline GaN ingots inthe liquid phase under a pressure of 12 to 20 kbars (12 to 20×10⁸ Pa)and at a temperature of between 1400 and 1700° C. These conditions aredifficult to implement, however, in the course of mass production. Inaddition, they only produce crystals having a maximum diameter of 18 mm.

Other teams of researchers have also worked on methods for growingingots in liquid phase at reduced pressure (less than 2 bars (2×10⁵ Pa))and at a temperature of 1000° C. The diameter of the crystals obtainedis larger, in the vicinity of 50 mm, but the crystalline qualityobtained is less satisfactory than in the previously mentioned method.

Finally, the article “Growth and characterization of GaN singlecrystals”, Balka et al., Journal of Crystal Growth, Vol. 208, January2000, pp. 100-106, discloses the growth of monocrystalline GaN bysublimation. The manufacturing conditions used are a pressure of lessthan 1 bar (10⁵ Pa) and a temperature of from 1000 to 1200° C. Thecrystal quality is very good but the size of the crystal is 3 mm, whichis clearly inadequate for the intended applications in the semiconductorfield.

At the present time, there is no monocrystalline gallium nitride oraluminum nitride on the market, in a massive form, of good quality,having sufficient diameters and at a reasonable price. In order toresolve this problem, one notes in the literature a number of attemptsat manufacturing substrates comprised of monocrystalline, free-standinggallium nitride by thick heteroepitaxy and then eliminating the epitaxysubstrate. This thick epitapy or hydride vapor phase epitapy (known tothe skilled artisan under the acronym HVPE or “hydride vapor phaseepitapy”) consists of producing epitaxial growth of GaN on diversesubstrates between 1000° C. and 1100° C. at atmospheric pressure with aview to obtaining a layer of GaN of several tens or hundreds of microns.This technique is advantageous in that it enables one to obtain a goodcrystal quality and in that it is not necessary to face or to cut theingots of crude material as in the aforementioned prior art. However,the GaN plates obtained in this fashion have many residual stresses andtensions connected with heteroepitaxy.

Several methods are distinguished according to the nature of the epitaxysupport substrate and the technique used to remove the substrate. Thus,according to the article “Physical properties of bulk GaN crystals grownby HVPE”, Melnik et al., MRS Internet Journal of Nitride SemiconductorResearch, Vol. 2, Art. 39, a method for growing GaN monocrystals usingHVPE on a substrate comprised of monocrystalline silicon carbide (SiC)with removal of the substrate by reactive ionic etching (known to theperson skilled in the art under the acronym RIE according to the Englishexpression “reactive ionic etching”). However, removal of this SiCsubstrate is very time-consuming because it is chemically very inert.

Also, according to the article “Large free-standing GaN substrates byhydride vapor phase epitaxy and laser-induced lift-off,” Kelly et al.,Jpn. J. Appl. Phys., Vol. 38, 1999, a method for growing GaN by HVPEepitaxy on a sapphire substrate and subsequent removal of the substrateby laser is known (known by the English terminology, “laser-inducedlift-off”). Implementing this removal technique is delicate for treatinglarge surfaces because the laser beam scanning is long.

It is also possible to remove the sapphire substrate by mechanicalpolishing but this method is likewise time-consuming and furtherpresents the risk of breakage of the GaN layer at the time of removal ofthe substrate that releases the stresses.

In other respects, the article “Preparation of large free-standing GaNsubstrates by hydride vapor phase epitaxy using GaAs as a startingsubstrate,” Motoki et al., Jpn. J. Appl. Phys., Vol. 40 (2001), pp.L140-L143 describes a method for growing GaN on a substrate comprised ofgallium arsenide (GaAs) by HVPE and then chemical dissolution of thesubstrate. This technique enables easy removal of the substrate,however, the latter is lost, which is less of an advantage from aneconomic point of view.

Other attempts have also been made by implementing a technique comprisedof growing GaN or aluminum nitride (AlN) on a supporting substrate ofsilicon (Si {111}) by HVPE and then removing the supporting substrate bychemical etching. This technique has the same drawbacks as thosementioned previously.

Finally, according to U.S. Pat. No. 6,176,925, U.S. Patent applicationsPublication Nos. 2001/0006845 and 2001/00022154 and European PatentApplication No. 1,045,431, methods are known for obtaining a thick layerof gallium nitride by epitaxial techniques on a seed layer which itselfhas been obtained by epitaxy. However, none of these four documentsmention the possibility of placing a nucleation layer on a support bymolecular adhesion bonding.

The important points for realizing free-standing substrates are on theone hand the capacity of realizing thick epitaxy; that is, at least 100microns while having good crystal quality and on the other hand easyseparation of the thick layer from its epitaxy support. The presentinvention now remedies the aforementioned drawbacks while respectingthese important points.

SUMMARY OF THE INVENTION

The invention relates to methods for manufacturing a free-standingsubstrate made of a semiconductor material, preferably of amono-crystalline semiconductor material that is most preferably of awide band gap material, such as gallium nitrate (GaN), aluminum nitrate(AlN), diamond, or silicon carbide (SiC). Wide band gap materials areknown in the art and generally have a band gap value above 1.5 eV.

This method comprises providing a first assembly by bonding a nucleationlayer of a first material to a support of a second material, with aremovable bonding interface being defined and located between facingsurfaces of the nucleation layer and the support; growing, by epitaxy onthe nucleation layer, a substrate of a layer of a third material to forma second assembly with the substrate attaining a sufficient thickness tobe free-standing, with at least the substrate being heated to anepitaxial growth temperature, wherein the third material is a wide bandgap material; and selecting the coefficients of thermal expansion of thesecond and third materials to be different from each other by a thermalexpansion differential, determined as a function of the epitaxial growthtemperature or subsequent application of external mechanical stresses,such that, as the second assembly cools from the epitaxial growthtemperature, stresses are induced in the removable bonding interface tofacilitate detachment of the nucleation layer and the substrate from thesupport.

Another method comprises preparing a first assembly that includes arelatively thinner nucleation layer of a first material, a support of asecond material, and a removable bonding interface defined betweenfacing surfaces of the nucleation layer and support;

growing, by epitaxy on the nucleation layer, a substrate of a relativelythicker layer of a third material, to form a second assembly with thesubstrate attaining a sufficient thickness to be free-standing whilepreserving the removable character of the bonding interface, with atleast the substrate being heated to an epitaxial growth temperature; andselecting the coefficients of thermal expansion of the second and thirdmaterials to be different from each other by a thermal expansiondifferential, determined as a function of the epitaxial growthtemperature or subsequent application of external mechanical stresses,such that, as the second assembly cools from the epitaxial growthtemperature, stresses are induced in the removable bonding interface tofacilitate detachment of the nucleation layer from the substrate.

In these methods, the coefficients of thermal expansion of the secondand third materials are preferably selected to be sufficiently differentfrom each other so that the nucleation layer and substrate becomedetached as the second assembly cools to ambient from the epitaxialgrowth temperature. Alternatively, a thermal treatment can be applied toraise stresses in the removable bonding interface to assist in thedetachment of the nucleation layer and substrate. Also, an externalstress can be applied to assist in the detachment of the nucleationlayer and substrate. The external stress includes mechanical stressesapplied from a jet of fluid or a blade, such as a guillotine.

The substrate is preferably a monocrystalline material that can bedeposited at least in part by hydride vapor phase epitaxy (HPVE).Advantageously, the nucleation layer can be applied onto the substrateby direct bonding with molecular adhesion, and the removable bondinginterface is located between the facing surfaces of the nucleation layerand the substrate. If desired, the first assembly can be provided withat least one intermediate bonding layer positioned between thenucleation layer and the substrate. Alternatively, the first assemblycan be provided with two intermediate bonding layers, one positionedadjacent the nucleation layer and the other positioned adjacent thesubstrate, with the removable bonding layer created by opposedcontacting surfaces of the intermediate bonding layers. Preferably, oneof the intermediate bonding layers is a layer of silicon oxide (SiO₂) orsilicon nitride (Si₃N₄).

The method of the invention also contemplates creating the bondinginterface by effecting a treatment for augmenting the roughness of thefacing surface of at least one of the nucleation layer or the substrate.This can be done by chemical attack or etching. Also, the bondinginterface can be created by effecting a treatment for decreasinghydrophily of the facing surface of at least one of the nucleation layeror the substrate.

The epitaxial growing of the substrate includes initially providing afine nucleation layer on the nucleation layer in order to improve thecrystal quality of the deposited third material. The fine nucleationlayer may be provided by metal organic chemical vapor deposition (MOCVD)epitaxy or by molecular beam (MBE) epitaxy. If desired, the nucleationlayer can be eliminated so that the substrate becomes a free-standingstructure.

The nucleation layer can be formed by implantation of an atomic speciesinto a source substrate to a defined depth to form at an embrittled zonethat defines a boundary of the nucleation layer in the source substrate.The source substrate may be monocrystalline or polycrystalline andpreferably is carbide, silicon, sapphire, gallium nitride or aluminumnitride, generally in the form of a wide bandgap material. The preferrednucleation layer is a monocrystalline material of gallium nitride,silicon, silicon carbide, sapphire, diamond, or aluminum nitride. Oneadvantageous combination is a substrate of diamond and a nucleationlayer of diamond, silicon or silicon carbide.

The invention also relates to a semiconductor material made of arelatively thinner nucleation layer of a first material, a support of asecond material, and a removable bonding interface defined betweenfacing surfaces of the nucleation layer and support, and a substrate ofa relatively thicker layer of a third material to form an assemblywherein the substrate has a sufficient thickness to be free-standingwhile preserving the removable character of the bonding interface, withthe second and third materials having coefficients of thermal expansionthat are different from each other by a thermal expansion differential,determined as a function of the epitaxial growth temperature orsubsequent application of external mechanical stresses, such that,stresses are induced in the removable bonding interface to facilitatedetachment of the nucleation layer from the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, objects and advantages of the invention will emerge onreading the detailed description that follows. This description will bebetter understood when read with reference to the annexed drawings,wherein:

FIGS. 1-6 are representations illustrating different series ofsuccessive stages of the method according to the invention and theirvariants, with:

FIG. 1 illustrates a source material having an embrittled zone;

FIGS. 2A and 2B illustrates a source material that is subjected to aporosification treatment (FIG. 2A), followed by epitaxial growth ofanother material thereon (FIG. 2B);

FIGS. 3A, 3B and 3C illustrates the application of a nucleation layerfrom a source material onto a support (FIGS. 3A and 3B), and theresulting structure (FIG. 3C) after removal of the source material alongthe embrittled zone.

FIGS. 4A, 4B and 4C illustrates another application of a nucleationlayer from a source material onto a support (FIGS. 4A and 4B), and theresulting structure (FIG. 4C) after removal of the source material alongthe embrittled zone.

FIGS. 5A, 5B and 5C illustrates the deposition of a monocrystallinelayer onto the nucleation layer (FIG. 5A) prior to removal of thesupport (FIG. 5B) and nucleation layer (FIG. 5C); and

FIGS. 6A, 6B and 6C illustrates another application for the depositionof a monocrystalline layer onto the nucleation layer (FIG. 6A) prior toremoval of the support (FIG. 6B) and nucleation layer (FIG. 6C)

In these drawing figures it will be noted that the different layers arenot represented in their actual scale, especially regarding theirthicknesses, but are instead shown schematically to illustrate themethod steps and resulting structures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a method for manufacturing afree-standing substrate made of microcrystalline semiconductor material.A preferred aspect of this method includes the following constituentsteps:

transferring a thin nucleation layer onto a support by creating betweenthe two a removable bonding interface;

growing by epitaxy on the thin nucleation layer, a microcrystallinelayer of material intended to comprise the substrate until it attains asufficient thickness to be free-standing, while preserving the removablecharacter of the bonding interface,

choosing the coefficients of thermal expansion of the material of thesubstrate and of the support material to be different from each otherwith a difference determined as a function of the temperature ofepitaxial growth and a possible application of external mechanicalstresses such that at the time of cooling the assembly, starting fromthe epitaxial growth temperature, the stresses induced by differentialthermal expansion between the support material and that of thesubstrate, combined, if required, with the application of exteriormechanical stresses, causing the removal of the nucleation layer and themonocrystalline substrate from the support at the level of the removablebonding interface.

The method according to the invention also has the followingadvantageous characteristics, taken in isolation or in combination:

the deposition of the substrate by epitaxy is realized at least in partby hydride vapor phase epitaxy (HVPE);

the thin nucleation layer is applied onto the support by direct bondingby molecular adhesion, the removable bonding interface being comprisedof the contact surface between the this nucleation layer and thesupport;

before applying the nucleation thin layer onto the support, a firstbonding interface is applied onto at least one of the two, the removablebonding interface being comprised of the surface contact between on theone hand the first bonding intermediate layer and on the other hand thesecond bonding intermediate layer or the nucleation thin layer or thesupport;

at least one of the bonding interfaces is a layer of silicon oxide(SiO₂);

at least one of the bonding interfaces is a layer of silicon nitride(Si₃N₄);

the bonding interface is made removable by applying a treatment foraugmenting the roughness of at least one of the two contact faces at thelevel of the bonding interface;

the treatment for augmenting the roughness of the surface is done bychemical attack;

the bonding interface is made removable by carrying out a treatment forreducing the hydrophily of at least one of the two faces in contact withthe bonding interface;

the bonding interface is made removable by a thermal treatment using athermal budget with a view to reducing the bonding energy between thetwo faces in contact with the bonding interface;

The method comprises growing by epitaxy a fine nucleation layer on thenucleation thin layer prior to growing by epitaxy the substrate. Thenucleation fine layer is created by metalo-organic chemical vapordeposition (MOCVD) epitaxy or by molecular beam epitaxy (MBE). Theexterior mechanical stresses applied at the time of removal are chosenfrom application of a jet of fluid, use of a blade or a guillotine. Themethod comprises the additional step of elimination of the nucleationlayer that remains integral with the substrate intended for comprisingthe free-standing substrate. Furthermore, prior to applying thenucleation thin layer onto the support, the nucleation layer is formedby implantation of atomic species to the interior of a source substrate,in the vicinity of a defined depth, in such a fashion as to define atthe depth a weakened or embrittled zone separating the nucleation layerfrom the rest of the source substrate.

The monocrystalline material comprising the free-standing substrate ispreferably a prohibited broadband material, such as gallium nitride(GaN) or aluminum nitride (AlN). The support is preferablymonocrystalline or polycrystalline and is chosen from silicon carbide,silicon, sapphire, gallium nitride or aluminum nitride. The nucleationthin layer is realized using a monocrystalline material chosen fromgallium nitride, silicon, silicon carbide, sapphire, diamond or aluminumnitride. The substrate is realized using diamond and the nucleationlayer is diamond, silicon or silicon carbide.

before transferring the thin nucleation layer onto the support, a firstintermediate bonding layer is applied onto at least one of the two, theremovable bonding interface being comprised of the contact surfacebetween on the one hand the first intermediate bonding layer and on theother hand the second intermediate bonding layer or the thin nucleationlayer or the support;

at least one of the intermediate bonding layer is a layer of siliconoxide (SiO₂);

at least one of the intermediate bonding layer is a layer of siliconnitride (Si₃N₄);

the bonding interface is made removable by effecting a treatment foraugmenting the roughness of at least one of the two faces in contact atthe level of the bonding interface;

the treatment for augmenting the roughness of the surface is carried outby chemical attack or etching;

the bonding interface is made removable by effecting a treatment fordecreasing the hydrophily of at least one of the two faces in contactwith the bonding interface;

the bonding interface is made removable by a thermal treatment using athermal budget with a view of reducing the bonding energy between thetwo faces in contact at the level of the bonding interface;

the method comprises, prior to the epitaxial growth of the substrate,growing by means of epitaxy a fine nucleation layer on the nucleationthin layer;

the fine nucleation layer is produced by means of metal organic chemicalvapor deposition (MOCVD) epitaxy or by molecular beam epitaxy (MBE);

the external mechanical stresses applied at the time of removal arechosen from application of a jet of fluid, use of a blade or aguillotine;

the method comprises the step of supplementary elimination of thenucleation layer that remains integral with the substrate intended toform the free-standing substrate;

prior to transferring the thin nucleation layer onto the support, thenucleation layer is formed by implantation of atomic species at theinterior of a source substrate, in the vicinity of a defined depth, insuch a fashion as to define at the depth an embrittled zone separatingthe nucleation layer from the rest of the source substrate;

the support is monocrystalline or polycrystalline and is chosen fromsilicon carbide, silicon, sapphire, gallium nitride or aluminum nitride;

the monocrystalline material comprising the free-standing substrate ispreferably a wide bandgap material. Any wide bandgap material having a abandgap value above 1.5 eV can be used. Specifically preferred widebandgap materials include gallium nitride (GaN); aluminum nitride (AlN);or silicon carbide (SiC);

the thin nucleation layer is realized using a monocrystalline materialchosen from gallium nitride, silicon, silicon carbide, sapphire, diamondor aluminum nitride;

the substrate is realized in diamond and the nucleation layer isrealized in diamond, silicon or silicon carbide.

The method in its entirety will first be briefly described. This methodcomprises:

bonding a seed layer or “nucleation layer” on a mechanical support bymeans of a removable bonding interface, then

-   -   growing, by epitaxy, on the nucleation layer, a substrate of the        material comprising the substrate that is to be obtained, by        thus forming a stack of layers and then    -   removing the substrate and the nucleation layer, from the        mechanical support, at the level of the removable bonding layer,        in particular by imposing thermal stresses associated to the gap        between the thermal expansion coefficients of the different        layers constituting the stack, and eventually mechanical        stresses.

This method will now be described in more detail.

FIG. 1 represents a first variant for obtaining the nucleation layer.The purpose of the method according to the invention is the manufactureof the free-standing substrates in a monocrystalline semi-conductormaterial, namely using a material having a wide prohibited band, andparticularly those substrates comprised of gallium nitride (GaN) oraluminum nitride (AlN), or under certain conditions even diamond.

In order to prepare the nucleation layer, a source substrate 1 ischosen, whose nature and crystal lattice parameters enable thesubsequent growth by epitaxy thereupon of a thick monocrystalline layerthat will constitute a free-standing substrate for the nucleation layer.Consequently, one can choose in a particularly appropriate fashion asthe source substrate 1, a monocrystalline material such as galliumnitride (GaN), silicon (Si); silicon carbide (SiC), sapphire, diamond oraluminum nitride (AlN). It should be noted that when the free-standingsubstrate is made of diamond, the nucleation layer should preferablyalso be made of diamond, silicon or silicon carbide.

An atomic species implantation operation 3 is carried out on one of theflat surfaces of this substrate 1, called the frontal face 2.Implantation of an atomic species is defined as any bombardment ofatomic, molecular, or ionic species capable of introducing the speciesinto a material with a maximum of concentration of the species in thematerial, the maximum being at a defined depth with respect to thebombarded surface 2. The atomic, molecular or ionic species isintroduced into the material using an energy equally distributed arounda maximum. The implantation of the atomic species into the sourcesubstrate 1 can be done, for example, using an ion beam implanter or aplasma immersion implanter. Preferably, this implantation is done byionic bombardment. Also preferably, the ionic species implanted ishydrogen. Other ionic species can be advantageously used alone or incombination with hydrogen such as rare gases (e.g., helium).

This implantation has the effect of creating in the volume of the sourcesubstrate 1 and at an average depth of ion penetration, an embrittledzone 4 separating the substrate 1 into two parts. On this subject,reference is made to the literature concerning the well known SMART-CUT®method.

The embrittled zone 4 formed in this fashion delimits the one layer 5corresponding to the upper part of the substrate 1 and extending fromthe bombarded surface 2 to the embrittled zone 4 and a lower partcorresponding to the rest of the source substrate 1. This layer 5 willlater constitute the nucleation layer.

By way of example, the energy of implantation that can be obtained usingthe equipment currently available in micro-electronics is such that themaximal thickness of the nucleation layer 5 is of the order of 0.5μ to1.5μ. Using more powerful implanters it is possible to obtain a thickernucleation layer 5. Thus, the skilled artisan can select the appropriatebombardment ions and energy to achieve the desired thickness of layer 5.

FIGS. 2A and 2B represent a variant for obtaining the nucleation layer.The source substrate 1 used is identical to that described above. Aso-called porosification treatment 6 is effected on the frontal surface2′. By way of example, one can refer to EP 0 849 788, which describessuch a process. Then one proceeds with the epitaxial growth on thisfrontal surface 2′ of a supplemental layer 5′ of a material preferablyidentical to that of the source substrate 1. The porosificationtreatment thus enables forming a layer or embrittled zone 4′ imbeddedbetween the source substrate 1 and the layer 5′; the latter constitutingthe future nucleation layer.

FIGS. 3A to 3C represent the application of the nucleation layer 5 onthe support 7. In these figures, as well as in FIGS. 4 to 6, for thesake of simplification, only the nucleation layer showing numeralreference 5 is represented; in other words, that one obtained by themethod represented in FIG. 1. However, it is quite obvious that it couldalso be the layer referenced using 5′ and obtained by the porosificationmethod described in conjunction with FIGS. 2A and 2B or by any othersimilar method.

In a first variant embodiment represented in FIG. 3B, the bondingbetween the nucleation layer 5 and the support 7 is made by molecularadhesion. These bonding techniques are known to the skilled artisan andare described, for example, in the paper by Gosële, “Semiconductor waferbonding”, Sciences and Technology, Q. Y. Tong, U. Gosële, WileyInterscience Publications.

In a second variant embodiment, represented by FIGS. 4A and 4B, thebonding is done by application of a first intermediate bonding layer 8on the nucleation layer 5 and a second intermediate bonding layer 8′ onthe support 7, then bonding of the two intermediate layers 8 and 8′ oneach other by molecular adhesion as hereinbefore described.

Finally, in a third variant embodiment (not represented in the figures)a single intermediate bonding layer 8′ is applied on the support 7 andthen bonded on top of the nucleation layer 5 or, inversely, a singleintermediate bonding layer 8 is applied on the nucleation layer 5 and itis then bonded on top of the support 7.

These bonding layers 8, 8′ are of a thickness in the area of 0.5μ andare advantageously layers of oxide, for example, SiO₂ or nitride, forexample Si₃N₄, deposited by chemical vapor deposition. It is alsopossible to intercalate intermediate bonding layers 8, 8′ of differenttypes, for instance, one layer of oxide and one layer of nitride,between the support 7 and the nucleation layer 5.

The support 7 and the nucleation layer 5 are then assembled as shown inFIGS. 3B and 4B by way of a bonding interface referenced by numeral 9.The term “bonding interface” is defined as the contact surface betweentwo facing surfaces, assembled with each other by bonding. According tothe different cases mentioned above, it can be either the contactsurface between the frontal surface 2 of the nucleation layer 5 and thefrontal face 70 of the support 7 (see FIGS. 3A and 3B) or the contactsurface between the respective frontal surfaces 80, 80′ of the twointermediate bonding layers 8 and 8′ (see FIGS. 4A and 4B).

Finally, when one single intermediate bonding layer is applied betweenthe nucleation layer 5 and the support 7, the bonding interface 9 isthen the contact surface between the intermediate layer and, dependingon the situation, the frontal face 2 of the nucleation layer 5 or thefrontal face 70 of the support 7 which was bonded on the intermediatelayer.

According to an important characteristic of the invention, this bondinginterface 9 is removable. This means that a treatment is effected priorto the molecular adhesion bonding step, which is intended to reduce thebonding energy level at the bonding interface 9 in such a fashion as tobring it to a level lower than that obtained by normal bonding.

In the course of the description and the claims, the expression “normalbonding” is defined as an operation comprising classical bonding bymolecular adhesion of two surfaces against each other after normalpreparation of the surfaces; in other words, cleaning in baths ofchemical products then thermal annealing; for more information on thissubject see the following publications: C. Maleville et al.,Semiconductor wafer bonding, Science Electrochemical Society ProceedingSeries, Permington, N.J. (1998) and O. Rayssac et al. “Proceeding of the2^(nd) International Conference on Materials for Microelectronics,” IOMCommunications, p. 183, 1998.

Of course, the value of this bonding energy is a function of the natureof the materials in contact along the bonding interface, of thetemperature at which the molecular adhesion bonding is effected, and ofthe temperature at which the thermal annealing is effected. By way ofpurely illustrative example, in the case of bonding of a layer of SiO₂to another layer of SiO₂, the bonding energy between the two layers ofSiO₂ is in the area of 100 mJ/m² for bonding done at ambient temperatureand after normal preparation of the surfaces and can attain 1 to 2 J/m²after annealing treatments between 400 and 1100° C. After treatmentintended to reduce the bonding energy level, for example by rougheningas described above, all of the other parameters being otherwiseidentical, the roughness is in the area of 0.625 nm RMS and the bondingenergy after the annealing cycle at 100° C. is in the area of only 500mJ/m². Essentially identical values are obtained using SiO₂/Si bonding.At the time of removal, disassembly is necessarily effected in the planeof the bonding interface 9 and not irregularly along a fracture linethat would at times extend into one of the opposing surfaces or into theopposing face, or between the two.

Various examples of treatment methods enabling reduction of the bondingenergy level and making the bonding interface 9 removable will now bedescribed. A first method consists of increasing the roughness of atleast one of the two faces in contact. This increase of the roughnesscan be done locally by chemical attack or etching using hydrofluoricacid (HF), for instance; see the article by O. RAYSSAC et al., forexample.

A second method consists in reducing the hydrophilicity of the surfacesto be brought into contact, prior to the actual bonding by chemicalcleaning using methods, for example, as described in the previouslymentioned article by C. Maleville.

A third method for obtaining a removable bonding interface consists inreducing the thermal budget normally sufficient to achieve bondingenergies currently obtained by standard bonding. The thermal budgetcorresponds to the temperature of a thermal treatment multiplied by theduration of the treatment.

Finally, it should be noted that it is possible to utilize any of theaforementioned methods alone or in combination.

As concerns the support 7, this plays essentially a mechanical supportrole. It is advantageously chosen from silicon carbide, silicon,sapphire, gallium nitride or aluminum nitride.

Then the nucleation layer 5 is removed from the rest of the sourcesubstrate 1 along the embrittled zone 4 (see FIGS. 3B and 3C or 4B and4C). The exposed top surface of the nucleation layer 5 is indicated bythe numerical reference 50. In order to allow detachment along theembrittled zone 4 and not along the bonding interface 9, it is necessarythat the embrittled zone 4 has a mechanical strength that is lower thanthat of the bonding interface 9.

In the case where the embrittled zone 4 is formed by hydrogenimplantation, detachment is effected either solely under the action ofthe application of an appropriate thermal budget by heating the layerstack formed at a sufficient temperature so as to induce detachment(typically 500° C. for silicon and 900° C. for silicon carbide), or bythe application of external mechanical stresses with or without thejoint application of a thermal budget. It should be noted that thethermal budget applied must, however, be limited in such a fashion as toconserve the removable character of the bonding interface 9.

Application of a mechanical stress can consist in exerting a bendingand/or traction force on the posterior part of the source substrate 1 orintroducing at the embrittled zone 4 a blade or a jet of fluid (gas orliquid), for example. It can also take the form of application ofshearing or ultrasound forces. The external mechanical stresses can alsobe of electrical energy origin deriving from the application of anelectrostatic or electromagnetic field. Finally, the external mechanicalstresses can also be of thermal energy origin deriving from theapplication of an electromagnetic field, an electron beam,thermoelectric heating, a cryogenic fluid, a super-cooled liquid, etc.

After detachment along embrittled zone 4, a stack comprised of themechanical support 7 and the nucleation layer 5 is obtained, betweenwhich one (or a plurality) of intermediate bonding layer(s) 8, 8′ (seeFIGS. 3C or 4C) may be intercalated. A finishing operation can beoperated on the exposed top surface 50 of the nucleation layer 5 inorder to improve the compatibility of the surface with the subsequentepitaxial growth. This finishing operation can be done by polishing,etching or thermal treatment and, in the last-mentioned case, it isassured that the application of the additional thermal budget does notdestroy the removable character of the bonding interface 9.

Other techniques for obtaining the nucleation layer 5 on the support 7are well known to the person skilled in the art and can also beutilized. For example, one technique derived therefrom can be cited,which enables obtaining substrates of the type known to the personskilled in the art under the acronym BESOI or ‘bond an etch back siliconon insulator’ or also BSOI, ‘bonded SOI’. These techniques consist inbonding the source substrate 1 directly onto the support 7 and thenproceeding with physical removal of the back of the source substrateeither by polishing techniques or by chemical etching techniques until alayer 5 of the desired thickness is obtained. In most cases, thenucleation layer will be relatively thin compared to the monocrystallinesupport substrate. The relative thicknesses can be selected as neededfor the intended application of the structure, and these are well knownto the skilled artisan.

As illustrated in FIGS. 5A and 6A, a relatively substrate 10 of themonocrystalline substrate that one wishes to obtain can be firstdeposited on the exposed upper surface 50 of the nucleation layer 5.Thus, a stack (reference 11) is obtained.

Advantageously, this substrate layer 10 deposit is effected by epitaxyand at least partly by means of hydride vapor phase epitaxy (known tothe person skilled in the art under the acronym HVPE or ‘hydride vaporphase epitaxy’). This deposit is effected at a temperature between 1000and 1100° C., preferably 1050° C. Care is taken to maintain thistemperature in a range of values allowing preservation of the removablecharacter of the bonding interface 9. This deposit is continued untilachieving a sufficient thickness that the layer 10 is ultimatelyfree-standing when it is removed from the support 7. The method ofrealizing this epitaxy, the parameters and the respective orientation ofthe nucleation layer and the substrate are known to the skilled artisan.

According to one embodiment of the invention represented in FIGS. 5A and5B for the sake of simplification but which could also be done using themethod variant represented in FIGS. 6A to 6C, it is also possible toproceed, prior to deposit of the substrate 10, with a growth phase bymeans of epitaxy of a fine nucleation layer 12. This can be done usingthe same material as that used subsequently for the realization of thesubstrate 10 but not necessarily identical to that of the nucleationlayer 5. This step can be advantageous for improving the crystal qualityof the substrate 10.

In this instance, and particularly for GaN, epitaxy of this fine layercan be realized by metal organic chemical vapor deposition (known to theskilled artisan under the acronym MOCVD) or by molecular beam epitaxy(known to the skilled artisan under the acronym MBE). It is alsopossible to use lateral growth techniques known to the skilled artisanunder the acronym ELOG or epitaxial lateral over-growth.

According to another embodiment, the material used to form the finenucleation layer 12 can also be different from that used for thesubstrate 10 and for the nucleation layer 5. By way of example, a fineepitaxial layer of AlN can be deposited on a nucleation layer of SiCprior to growth of a substrate of GaN. The formation techniques used forthe fine epitaxial layer are identical to those hereinbefore described.

As illustrated in FIGS. 5B and 6B, the support 7 and the intermediatebonding layer 8′, if present, will be removed from the rest of the stackcomprised of the nucleation layer 5 and the substrate 10 and, whenpresent, intermediate bonding layer 8. This removal is done along theremovable bonding interface 9.

The coefficients of thermal expansion are fixed values established for agiven material. However, the expansion of the material and its elasticenergy depend on its thickness. In a stack of layers, the behavior ofthe different layers is dictated in a first approximation by thethickest layer(s). In the present case, the nucleation layer 5 has athickness of several microns, while the monocrystalline layer 10 is morein the area of 100μ or even 200μ and the support is at least 300μ thick.Consequently, in the first approximation the thermal expansioncoefficients of the support 7 and those of the monocrystalline layer 10are taken into account in order to predict the removal behavior of thesetwo layers. At the second level, the presence and the nature of theintermediate bonding layer(s) can be significant for the distribution ofthe stresses in the structure considered.

In other words, if as a result of the nature of the materials chosen forthe support 7 and the monocrystalline layer 10, the difference betweentheir coefficients of thermal expansion is significant, then removal canbe achieved naturally, along the removable bonding interface 9, when thetemperature of the stacking 11 decreases, after epitaxy realized between1000 and 1100° C. When, on return of the stack 11 to ambient temperature(around 20 to 25° C.) require additional application of exteriormechanical stresses. These stresses are identical to those mentionedabove for the detachment along the embrittled zone 4.

Finally, elimination of the nucleation layer 5 is effected (see FIGS. 5Cand 6C) by polishing, ionic etching or by attack using a chemicalsolution, for example. The choice of the technique used is a function ofthe nature of the material of the layer 5 and is generally known to theskilled artisan.

Later, it is also possible to realize a finishing step comprisingremoval of several tens of microns from the part of the monocrystallinelayer 10 that is situated in contact with the nucleation layer 5. Thesupport 7 can also be recovered and recycled.

EXAMPLES

Four exemplary embodiments of the method of the invention are nowdescribed in detail.

Example 1 Manufacturing of a free-standing GaN substrate using anintermediate stack comprising a silicon Si support, two intermediateSiO₂/SiO₂ bonding layers and a monocrystalline GaN nucleation layer.

A monocrystalline gallium nitride (GaN) nucleation layer 5 is formed ina solid source substrate 1 of solid GaN by ion implantation. Thisnucleation layer 5 is bonded onto a silicon mechanical support 7 by twointermediate bonding layers 8, 8′ of SiO₂ bonded together along abonding interface 9.

Prior to being assembled together, the two intermediate layers 8, 8′undergo mechano-chemical planarization and surface treatment intended toincrease roughness of the opposing surfaces 80, 80′ (e.g., a treatmentwith hydrofluoric acid HF or with a chemical cleaning solution known tothe skilled artisan the name SC1 and comprised principally of ammoniadiluted with hydrogen peroxide H₂O₂) of the opposing surfaces 80, 80′).

Then the nucleation layer 5 is detached from the rest of the substrate 1along the embrittled zone 4 using a thermal treatment done at 900° C.

Thermal treatment for stabilization of the bonding interface 9 is doneat 950° C. over 2 hours; in other words, at a temperature slightlyhigher than that currently used for bonding of this type. The bondinginterface 9 is thus removable.

Then a surface finishing step is done on the exposed surface 50 of thenucleation layer 5.

One then proceeds with rapid growth of a GaN layer 10 using HVPE at1050° C. At this temperature, the bonding interface 9 is slightlyreinforced but has, however, bonding forces greatly lower than thestandard bonding energies (that is, around 1 to 2 J/m²).

When the GaN attains a thickness of 200 m; that is a thicknesssufficient to be ultimately free-standing, deposition is stopped and thestack of the layers obtained is brought to ambient temperature.

The silicon support 7, whose thickness is greater than 300 m is chosenin order to have a coefficient of thermal expansion of 2.5×10⁻⁶/K,whilst that of the GaN epitaxial substrate 10 is in the area of5.6×10⁻⁶/K.

Consequently, the mechanical strength of the stack thus formed is lowand disassembly along the bonding interface 9 is done spontaneously whenthe temperature of the stack, which had reached 1050° C. during theepitaxy process, decreases.

Elimination of the SiO₂ layers, the GaN nucleation layer 5, or evenseveral micrometers from the GaN substrate 10 is effected by finishingpolishing in order to give it the characteristics of a wafer.

Example 2 Manufacturing of a free-standing AlN substrate using anintermediate stack comprising a silicon Si support, intermediateSiO₂/SiO₂ bonding layers and a monocrystalline AlN nucleation layer.

One proceeds in identical fashion as described for Example 1, exceptthat the solid gallium nitride is replaced by aluminum nitride (AlN) andthat the different AlN layers have a coefficient of thermal expansion inthe area of 4.15×10⁻⁶/K, while the coefficient of thermal expansion ofthe silicon forming the support is 2.5×10⁻⁶/K.

Removal is done likewise naturally by lowering the temperature of thestack.

Example 3

Manufacturing of a free-standing GaN Substrate using an intermediatestack comprising a silicon support, a single intermediate bonding layerof SiO₂ and a silicon Si {111} nucleation layer.

A nucleation layer 5 of silicon {111} is formed in a source substrate 1of the same type by ion implantation. This nucleation layer 5 is bondedonto a silicon mechanical support 7 by an intermediate bonding layer 8of SiO₂ obtained by thermal oxidation of the top surface of the sourcesubstrate 1. The bonding interface 9 is disposed between the top surface80 of the SiO₂ layer and the frontal surface 2 of the nucleation layer5.

This bonding interface 9 is treated as in Example 1 so as to beremovable.

Then the nucleation layer 5 is removed from the rest of the sourcesubstrate 1 along the embrittled zone 4.

A finishing step is then operated on the exposed surface 50 of thenucleation layer 5.

Then fast growth of a GaN layer 10 by HVPE at 1050° C. is realized. Atthis temperature, the bonding interface 9 is slightly reinforced buthas, however, lower than normal bonding forces.

When the GaN attains a thickness of 200 m; that is, a thicknesssufficient so as to be ultimately free-standing, deposition is stoppedand the stack of layers is brought to ambient temperature.

The silicon support 7, whose thickness is greater than 300 m is chosento have a coefficient of thermal expansion of is 2.5×10⁻⁶/K, whilst thatof the substrate of epitaxial GaN is in the area of is 5.6×10⁻⁶/K.

Consequently, the mechanical strength of the stack so formed is low andremoval along the bonding interface 9 is done spontaneously when thetemperature of the stack, which had reached 1050° C. during epitaxy,decreases.

One then proceeds with the elimination removal of the Si{111} nucleationlayer 5 as described in Example 1.

Example 4

Manufacturing of a free-standing GaN substrate using an intermediatestack comprising a silicon support, two intermediate SiO₂/SiO₂ bondinglayers, a monocrystalline SiC nucleation layer and a supplementary fineepitaxial monocrystalline GaN layer.

A nucleation layer 5 of monocrystalline silicon carbide (SiC) is formedin a source substrate 1 of solid SiC by ion implantation. Thisnucleation layer 5 is bonded onto a silicon mechanical support 7 by twointermediate bonding layers 8, 8′ of SiO₂ bonded to each other along abonding interface 9.

This bonding interface 9 is treated as described in Example 1 in such amanner as to be removable and the nucleation layer 5 is detached fromthe rest of the substrate 1.

A finishing step is then operated on the exposed surface 50 of thenucleation layer 5.

The a fine layer 12 of GaN having a thickness of less than 3 or 4 m isdeposited by epitaxial growth by MOCVD. This deposit is done eitheruniformly over the entire surface of the nucleation layer or locally toachieve the effects of lateral growth (ELOG). Then the layer stack thusformed is allowed to cool until it reaches ambient temperature (around20° C.).

Then fast growth of a GaN layer 10 by HVPE at 1050° C. is realized. Atthis temperature, the bonding interface 9 is slightly reinforced buthas, however, lower than normal bonding forces (around 2 J/m²).

When the GaN attains a thickness of 200 m; that is, a thicknesssufficient so as to be ultimately free-standing, deposition is stoppedand the stack of layers is brought to ambient temperature.

The silicon support 7, whose thickness is in the area of 300 to 400 m ischosen to have a coefficient of thermal expansion in the area of2.5×10⁻⁶/K, while that of the substrate of epitaxial GaN is in the areaof is 5.6×10⁻⁶/K.

Consequently, the mechanical strength of the stack so formed is low andremoval or disassembly along the bonding interface 9 is donespontaneously when the temperature of the stack decreases.

A finishing step is then operated as described in Example 1.

Example 5

Manufacturing of a free-standing diamond substrate using an intermediatestack comprising a silicon Si support, two intermediate SiO₂/SiO₂bonding layers and a diamond nucleation layer.

A nucleation layer 5 of diamond is formed in a source substrate 1 ofhigh crystal quality monocrystalline diamond by ion implantation. Thisnucleation layer 5 is bonded onto a silicon mechanical support 7 by twointermediate bonding layers 8 of SiO₂.

The bonding interface 9 is disposed between the two surfaces 80, 80′ ofthe SiO₂ layers. It is treated as described in Example 1 so as to beremovable.

Then the nucleation layer 5 is removed from the rest of the substrate 1along the embrittled zone 4; one then proceeds with a finishing step ofthe exposed surface 50.

Then fast growth of a diamond layer 10 by CVD (chemical vapordeposition) at between 800 and 1000° C. is realized. At thistemperature, the bonding interface 9 is slightly reinforced but has,however, lower than normal bonding forces.

When the diamond attains a thickness of 200 m; that is, a thicknesssufficient so as to be ultimately free-standing, deposition is stoppedand the stack of layers is brought to ambient temperature.

The silicon support 7, whose thickness is greater than 300 m is chosento have a coefficient of thermal expansion of is 2.5×10⁻⁶/K, whilst thatof the substrate of diamond is in the area of is 1×10⁻⁶/K.

Consequently, the mechanical strength of the stack so formed isrelatively high and disassembly along the bonding interface 9 cannot bedone solely spontaneously when the temperature, which attained 800 to1000° C. during epitaxy, of the stack decreases. It is necessary toforce the separation by using a guillotine.

One then proceeds with the removal of the diamond nucleation layer 5 asdescribed in Example 1.

While illustrative embodiments of the invention are disclosed herein, itwill be appreciated that numerous modifications and other embodimentscan be devised by those of ordinary skill in the art. Features of theembodiments described herein can be combined, separated, interchanged,and/or rearranged to generate other embodiments. Therefore, it will beunderstood that the appended claims are intended to cover all suchmodifications and embodiments that come within the spirit and scope ofthe present invention.

1. A method for manufacturing a free-standing substrate made of asemiconductor material, which comprises: providing a first assembly bybonding of a nucleation layer of a first material to a support of asecond material at a bonding interface being defined and located betweenfacing surfaces of the nucleation layer and the support, wherein thenucleation layer is applied onto the support by direct bonding withmolecular adhesion; growing, by epitaxy on the nucleation layer, asubstrate of a layer of a third material to form a second assembly withthe substrate attaining a sufficient thickness to be free-standing, withat least the substrate being heated to an epitaxial growth temperature,wherein the third material is a wide bandgap material; and selecting thecoefficients of thermal expansion of the second and third materials tobe different from each other by a thermal expansion differential,determined as a function of the epitaxial growth temperature orsubsequent application of external mechanical stresses, such that, asthe second assembly cools from the epitaxial growth temperature,stresses are induced in the bonding interface to facilitate detachmentof the nucleation layer and the substrate from the support.
 2. Themethod of claim 1, wherein the wide bandgap material is silicon carbide.3. The method according to claim 1, wherein the coefficients of thermalexpansion of the second and third materials are selected to besufficiently different from each other so that the nucleation layer andsubstrate become detached as the second assembly cools to ambient fromthe epitaxial growth temperature.
 4. The method according to claim 1,which further comprises applying a thermal treatment to raise stressesat the bonding interface to assist in the detachment of the nucleationlayer and the substrate.
 5. The method according to claim 1, whichfurther comprises applying an external stress to assist in thedetachment of the nucleation layer and the substrate by increasing thestress between the materials at the bonding interface.
 6. The methodaccording to claim 1, wherein the substrate is a monocrystallinematerial deposited at least in part by hydride vapor phase epitaxy(HPVE).
 7. The method according to claim 1, wherein removal of thebonding interface is facilitated by effecting a treatment for augmentingthe roughness of the facing surface of at least one of the nucleationlayer or the support.
 8. The method according to claim 7, wherein thetreatment for augmenting surface roughness is carried out by chemicalattack or etching.
 9. The method according to claim 1, wherein removalof the bonding interface is facilitated by effecting a treatment fordecreasing hydrophilicity of the facing surface of at least one of thenucleation layer or the support.
 10. The method according to claim 1,wherein the epitaxial growing of the second material includes initiallyproviding a fine nucleation layer on the nucleation layer in order toimprove the crystal quality of the deposited third material of thesubstrate.
 11. The method according to claim 10, wherein the tinenucleation layer is provided by metal organic chemical vapor deposition(MOCVD) epitaxy or by molecular beam (MBE) epitaxy.
 12. The methodaccording to claim 1, which further comprises eliminating the nucleationlayer after detachment so that the substrate becomes a free-standingstructure.
 13. The method according to claim 1, which further comprises,prior to bonding the nucleation layer onto the support, forming thenucleation layer by implantation of an atomic species into a sourcesubstrate to a defined depth to form an embrittled zone that defines aboundary of the nucleation layer in the source substrate.
 14. The methodaccording to claim 13, wherein the source substrate comprises amonocrystalline or polycrystalline wide bandgap material.
 15. The methodaccording to claim 14, wherein the wide bandgap material of the sourcesubstrate is gallium nitride (GaN) or aluminum nitride (AIN).
 16. Themethod according to claim 1, which further comprises detaching thenucleation layer and the substrate from the support.
 17. The methodaccording to claim 16, which further comprises eliminating thenucleation layer after detachment so that the substrate becomes afree-standing structure.
 18. A method for manufacturing a free-standingsubstrate made of a semiconductor material, which comprises: providing afirst assembly by bonding of a nucleation layer of a first material toat least one intermediate bonding layer present on a support of a secondmaterial at a bonding interface being defined and located between facingsurfaces of the nucleation layer and the intermediate bonding layer ofthe support, wherein the nucleation layer is applied onto theintermediate bonding layer of the support by direct bonding withmolecular adhesion; growing, by epitaxy on the nucleation layer, asubstrate of a layer of a third material to form a second assembly withthe substrate attaining a sufficient thickness to be free-standing, withat least the substrate being heated to an epitaxial growth temperature,wherein the third material is a wide bandgap material; and selecting thecoefficients of thermal expansion of the second and third materials tobe different from each other by a thermal expansion differential,determined as a function of the epitaxial growth temperature orsubsequent application of external mechanical stresses, such that, asthe second assembly cools from the epitaxial growth temperature,stresses are induced in the bonding interface to facilitate detachmentof the nucleation layer and the substrate from the intermediate bondinglayer and the support.
 19. The method according to claim 18, wherein theintermediate bonding layer is positioned adjacent the nucleation layerand a second intermediate bonding layer is positioned adjacent thesupport.
 20. The method according to claim 19, wherein at least one ofthe intermediate bonding layers is a layer of silicon oxide or siliconnitride.
 21. A method for manufacturing a free-standing substrate madeof a semiconductor material, which comprises: forming a nucleation layerby implantation of an atomic species into a source substrate to adefined depth to form an embrittled zone that defines a boundary of thenucleation layer from a remainder of the source substrate; providing afirst assembly by bonding the nucleation layer of a first material to asupport of a second material, at a bonding interface being defined andlocated between facing surfaces of the nucleation layer and the support;detaching the nucleation layer at the embrittled zone to remove theremainder of the source substrate and provide the nucleation layerbonded to the support; growing, by epitaxy on the nucleation layer, asubstrate of a layer of a third material to form a second assembly withthe substrate attaining a sufficient thickness to be free-standing, withat least the substrate being heated to an epitaxial growth temperature;and selecting the coefficients of thermal expansion of the second andthird materials to be different from each other by a thermal expansiondifferential, determined as a function of the epitaxial growthtemperature or subsequent application of external mechanical stresses,such that, as the second assembly cools from the epitaxial growthtemperature, stresses are induced in the bonding interface to facilitatedetachment of the nucleation layer and the substrate from the support.22. The method according to claim 21, wherein the third material has aband gap value above 1.5 eV.